The present invention relates to the field of selective production of various radioisotopes. More particularly, the present invention relates to the production and separation of high specific activity copper isotopes, e.g., Cu67, from irradiated enriched Zn70 targets. This invention is the result of a contract with the Department of Energy(Contract No. W-7405-ENG-36).
Copper-67 (Cu67) is a radioisotope with significant potential for application in diagnostic and therapeutic nuclear medicine. As it decays to stable Zn67, with a 2.6 day half-life, it emits beta particles with energy maxima ranging from about 0.4 MeV to about 0.6 MeV. It also emits a gamma photon of 185 keV. Excellent research has shown that the beta particles are effective in treating various tumor types when the radioisotope is delivered to the disease site. The gamma photon is well-suited for imaging applications using the conventional Anger Gamma Camera so that Cu67 localized in a tumor, can be imaged using equipment typically available in a nuclear medicine facility. Despite its promise this radioisotope has failed to make significant impact in clinical nuclear medicine, primarily because the isotope is available only in a sporadic and limited supply. Current methods of production use high energy proton reactions on natural zinc targets at large accelerators operating only part of the year or in nuclear reactors on enriched Zn67 using high energy neutrons.
The possibility of early lung cancer detection using porphyrin as described by Cole et al. (U.S. Pat. No. 5,162,231) and the development of labeling porphyrin with metal ions (see, e.g., Mercer-Smith et al., Vol. 1 of Targeted Diagnosis and Therapy Series, J. T. Rodwell, ed., Marcel Dekker, New York, p. 317, 1988) and research into the possibility of therapy using Cu67 labeled monoclonal antibodies (see, e.g., de Nardo et al., J.Nucl. Med. 29, p.217, 1988) has generated increased interest in the availability of high specific activity Cu67 on a more consistent basis. Presently, these efforts have been severely restricted as a result of the sporadic supply of the Cu67 used for preparation of the porphyrin. Presently, accelerator-produced Cu67 is only available from Los Alamos National Laboratory (LANL) and Brookhaven National Laboratory (BNL) in large quantities about 6 to 8 months of the year. The present production methods at LANL and BNL rely on nonspecific spallation reactions which co-produce stable copper isotopes. These stable copper isotopes dilute the specific activity to levels which are acceptable for porphyrin/monoclonal antibody labeling research but are barely adequate for therapy. Reactor production via the Zn67 (n, p)Cu67 reaction is possible but production rates are too low to be financially feasible for long range treatment protocols. A number of possible methods of production were examined using the Los Alamos National Laboratory Van de Graff accelerator.
It has now been found that Cu67 can be produced at low energies, i.e., less than about 25 MeV. Thus, Cu67 may be produced throughout the year. As reaction pathways for production of stable copper isotopes do not exist in this reaction process, Cu67 with a higher specific activity may be produced than is currently possible with accelerator production.
Accordingly, it is an object of this invention to provide an improved method of producing Cu67 with a higher specific activity than previously available.
It is a further object of this invention to provide a method to produce Cu67 continuously throughout the year using a low energy proton induced (p, xcex1)reaction on enriched Zn70.
Still another object of this invention to provide a method of recovering and reusing the enriched Zn70 target material.
A still further object of this invention to provide a method of producing Cu67 essentially free of Cu64 in a low energy proton accelerator.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention provides a process of producing essentially copper64-free copper67 including irradiating an enriched zinc70 target with sufficient protons of an energy from about 10 MeV to about 25 MeV for time sufficient to produce copper67, and, separating copper67 from the irradiated target to yield an essentially copper64-free copper67 product. The present invention further provides a high specific activity copper67 product produced by the above described process, the copper67 product characterized as essentially free of copper64 and as having a specific activity greater than about at least 5 percent of theoretical value. The present invention further provides a process including separating enriched zinc70 from the irradiated target and recycling the separated enriched zinc70 into an enriched zinc70 target for subsequent irradiation.
In one embodiment of the invention, the separation of copper67 from the irradiated target includes dissolution of the proton-irradiated enriched zinc target in an acid solution of appropriate type and strength to form a first ion-containing solution, contacting the first ion-containing solution with a first anionic exchange resin whereby ions from the group consisting of zinc, copper, gallium, aluminum, cobalt and iron are selectively removed from the solution and ions from the group consisting of beryllium, nickel, and germanium remain in the solution, contacting the first anionic exchange resin with a second acid solution of appropriate type and strength capable of stripping the absorbed ions of copper, gallium, aluminum, cobalt, and iron from the first anionic exchange resin to form a second ion-containing solution, evaporating the second ion-containing solution for time sufficient to remove substantially all of the acid and water from the second ion-containing solution whereby a residue remains, dissolving the residue from the second ion-containing solution in a concentrated acid to form a third ion-containing solution to permit said ions to be absorbed by a first cationic exchange resin, contacting the cationic exchange resin with concentrated acid to selectively remove copper ions while any gallium, aluminum, cobalt and iron ions present remain absorbed by the first cationic exchange resin. Alternatively, the residue from the second ion-containing solution can be dissolved in a concentrated acid to form a third ion-containing solution to permit said ions to be absorbed by an anion exchange resin, and the third-ion containing solution contacted with an anionic exchange resin followed by stripping with concentrated acid to selectively remove copper ion while any gallium, aluminum, cobalt and iron ions present remain absorbed by the anion exchange resin.
The present invention concerns the proton irradiation, selective recovery of radioisotopes of copper and the enriched zinc target material, and fabrication of new targets from such recovered target material. Such a process can produce multi-millicurie quantities of such radioisotopes for use in the fields on nuclear medicine and/or nuclear chemistry.
In the process of the present invention, a copper67 product essentially free of any copper64 can be produced as the reaction with enriched zinc70 produces only copper67. This leads to a product that avoids the presence of a second radioisotope thus eliminating the unnecessary exposure to that radioisotope while using the targeted radioisotope. Further, as essentially only copper67 is produced, the specific activity of the product is naturally greater. As the specific activity of radioisotopes is effected by co-production of other stable or radioactive isotopes, high specific activities cannot be achieved using previous methods. Prior copper67 available had a specific activity of only about 1.5 percent of the theoretical maximum of about 755,000 curies per gram. The present process may achieve specific activities much higher, specific activities of greater than about 5 percent of theoretical maximum value, preferably greater than about 25 percent of theoretical maximum value, and more preferably greater than about 50 percent of theoretical maximum value.
By xe2x80x9cessentially copper64xe2x80x94freexe2x80x9d is meant that the ratio of copper64 copper67 is about 0.1 or less to 1, preferably about 0.01 or less to 1. In contrast, previous methods of making and recovering copper67 generally had copper64 to copper67 ratios of at least around 4 or 5 to 1.
As a starting material in the present process, an enriched Zn70 target is irradiated by protons of an energy of generally greater than 10 MeV and less than 25 MeV. To produce the desired quantities of Cu67 , the Zn70 target should be enriched to at least about 70 percent, preferably greater, should have a weight of from about 250 milligrams to about 500 milligrams and should be from about 0.5 mm in thickness to about 0.75 mm in thickness. This irradiation can be accomplished by inserting the target into any particle accelerator capable of producing proton beams of the desired energy and beam current. A beam current of greater than about 20 microamps, but generally less than about 500 microamps, is generally preferred to maximize production.
The proton-irradiated Zn70 target is dissolved in 6 M HCl prepared from concentrated, ultra pure, copper-free hydrochloric acid and 17.9 Meg-ohm water and the resultant solution adjusted to about 8-10 M HCl with concentrated, ultrapure, copper-free hydrochloric acid and contacted with an first anion exchange resin. The anion resin is preferably a strong basic resin and can be, for example, AG1-X8, available from Bio-Rad Laboratories. Generally the anions of zinc, copper, gallium, cobalt and iron will be absorbed on the resin while other ions including beryllium, nickel, and germanium if present will remain in the solution and can be effectively separated from the anion exchange resin. Residual cations in solution may be removed from the resin by washing or rinsing with 8-10 M HCl. The mesh size of the anion exchange resin can be from about 50 mesh to about 400 mesh, more preferably from about 100 mesh to about 200 mesh.
The absorbed anions of Cu67 and gallium can then be stripped from the resin by contacting the resin with and acid solution capable of removing such ions. Generally, the acid solution can be from 1 molar (M) hydrochloric acid to about 4 M hydrochloric acid, preferably about 2 M hydrochloric acid. The strip solution is evaporated to dryness, and dissolved in concentrated, ultrapure, copper-free hydrochloric acid. The dissolved residue is then contacted with a first cation exchange resin. The cation resin is preferably a strong acid resin and can be, for example AG50W-X8, available from Bio-Rad Laboratories. Generally the ions of gallium will be absorbed on the resin while the ions of Cu67 will remain in solution and can be effectively separated from the cation resin. The cation resin is then washed with concentrated, ultrapure, copper-free hydrochloric acid to remove residual Cu67 ions. All of the concentrated, ultrapure, copper-free hydrochloric acid fractions containing Cu67 ions are combined, evaporated to dryness, dissolved in dilute hydrochloric acid prepared from concentrated, ultrapure, copper-free hydrochloric acid and 17.9 Meg-Ohm water to form the product solution. Generally this dilute hydrochloric acid can be from about 0.01 M hydrochloric acid to about 1 M hydrochloric acid, preferably about 0.1 M hydrochloric acid. Alternatively, the 2 M hydrochloric acid strip solution may be evaporated to dryness, dissolved in concentrated, ultrapure, copper-free hydrochloric acid and contacted with a second anion exchange resin. The anion resin is preferably a strong basic resin and can be, for example, AG1-X8, available from Bio-Rad Laboratories. Washing with 10-15 resin volumes of concentrated, ultrapure, copper-free hydrochloric acid will recover all of the Cu67 product free of gallium ions. All of the concentrated, ultrapure, copper-free hydrochloric acid containing Cu67 ions are combined, evaporated to dryness, dissolved in dilute hydrochloric acid prepared from concentrated, ultrapure, copper-free hydrochloric acid and 17.9 Meg-Ohm water to form the product solution. Generally this dilute hydrochloric acid can be from about 0.01 M hydrochloric acid to about 1 M hydrochloric acid, preferably about 0.1 M hydrochloric acid.
The enriched zinc target material may be recovered from the first anion exchange resin with dilute nitric acid. Generally this dilute nitric acid can be from about 0.5 M nitric acid to about 4 M nitric acid, preferably about 2 M nitric acid. The recovered zinc target material may then be converted to the chemical form required for fabricating a new target. This may include electrochemical deposition of the metal or conversion to the oxide chemically.